SUMO-1 is a ubiquitin-like protein that can be conjugated to other proteins in a manner similar to ubiquitin conjugation. Our goals are to understand the function and regulation of SUMO-1 at a molecular level, with a particular focus on the mitotic roles of SUMO-1 and on how SUMO-1 controls the Ran GTPase activating protein RanGAP1. SUMO proteases are required for the processing of SUMO-1 prior to conjugation and for deconjugation of SUMO-1 from other proteins. Two SUMO proteases have been described in budding yeast, Ulp1p and Ulp2p/Smt4p. Ulp1p is concentrated near the nuclear periphery and interacts with nuclear pore components in two-hybrid assays. ULP1 is an essential gene, and temperature sensitive Ulp1p mutants arrest at the G2/M transition of the cell cycle. In mammals, there are at least seven SUMO protease family members. We have been particularly studying in one member of this family, SENP2. We found that full-length human SENP2 associates with nuclear pores in a manner similar to Ulp1 in yeast. This association occurs exclusively with the nuclear face of the pore and requires sequences near the N-terminus of SENP2. We also found that SENP2 binds specifically to Nup153, a nucleoporin localized to nucleoplasmic face of the nuclear pore, and that this association requires the same domain of SENP2 that mediates its targeting in vivo. Remarkably, a mutant SENP2 protein that is unable to bind Nup153 is significantly more effective in promoting deconjugation of SUMO-1-conjugated species, indicating that localization of SENP2 to the nuclear pore plays an important role in spatially restricting the activity of this enzyme. Ran-GTP has an important role in regulating the organization of the cell during both interphase and mitosis (see Z01 HD008740-01). Given this role, knowledge of the distribution of Ran regulators will be essential for understanding the control and function of this pathway. In metazoans, RanGAP1 is conjugated with SUMO-1. SUMO-1 modification causes RanGAP1 to associate with RanBP2, a large nuclear pore protein, and Ubc9, the E2 enzyme for SUMO-1 conjugation. We have examined the behavior of RanGAP1 during mitosis. We found that RanGAP1 associates with mitotic spindles and that it is particularly concentrated at foci near kinetochores. Association with kinetochores appeared soon after nuclear envelope breakdown and persisted until late anaphase, but it was lost coincident with nuclear envelope assembly in telophase. A mutant RanGAP1 protein lacking the capacity to be conjugated to SUMO-1 no longer associated with spindles, indicating that conjugation was essential for mitotic localization of RanGAP1. RanBP2 co-localized with RanGAP1 on spindles, suggesting that a complex between these two proteins may be involved in mitotic targeting of RanGAP1. Our findings have shown for the first time that SUMO-1 conjugation is required for mitotic localization of RanGAP1, and suggests that a major role of SUMO-1 conjugation to RanGAP1 may be the spatial regulation of the Ran pathway during mitosis.